The present invention relates to an electron beam apparatus for use with a wafer having patterns with a minimum line width of 0.1 micron or less for evaluating the wafer in a defect test, a line width measurement, an alignment accuracy measurement, and the like, and to a method of manufacturing devices with a high yield rate using the electron beam apparatus.
Conventionally, a defect detection, a CD measurement, a defect review, SEM, an alignment accuracy measurement, and the like have been conducted by known apparatuses which irradiate a sample such as a wafer with an electron beam to detect secondary electrons generated from the sample. Particularly, electron beams are widely used in a method of detecting defects of a sample by applying a charge to patterns on the sample, and measuring a resulting potential on the surface for the evaluation because such an evaluation cannot be performed on an optical basis.
For generating a potential contrast of a pattern on a sample in a conventional electron beam apparatus, it is known that an apparatus not provided with an energy filter for secondary electron beams has a low resolution, whereas an apparatus provided with such an energy filter can measure a potential with a low potential resolution. When the function of energy filter is given to an objective lens, a problem arises in that the resulting objective lens has a large aberration coefficient. Also, for correctly aligning a site of a sample under evaluation to the field of view of an electro-optical system, a sub-system is required for registration similar to a lithography system, giving rise to an additional problem that the overall apparatus is increased in size and complicated.
Additionally, in the conventional electron beam apparatus, a variety of problems have been left unsolved, for example, as follows:
(1) In regard to an E×B separator for separating a primary electron beam from a secondary electron beam, it is unknown how a desired accuracy is provided in a simple structure.
(2) Since the conventional electron beam apparatus involves large shot noise, a large beam current is required to provide a desired signal/noise ratio.
(3) There has been no electrostatic chuck which is capable of flatly chucking a convexly upwordly distorted wafer.
(4) In a region of a wafer in which a field of view of a primary optical system overlaps, some locations on the wafer could be dosed with an electron beam of intensity twice or four times higher, as the case may be, possibly giving rise to breakage of a gate oxidation layer of the wafer.
(5) The conventional electron beam apparatus experiences difficulties in ensuring a space near a lens located above an objective lens for disposing a deflector for scanning.
As has been known in the field of infrared detector, shot noise if2 can be expressed by:if2=2e·I0·Γ2·Δf when a current I0 is flowing through an electron gun. Under a condition in which an electron gun is operated under a temperature limited condition, Γ is 1.0, while under a condition in which an electron gun is operated under space charge limited condition, Γ is in a range from 0.1 to 1.0 (see R. A. Smith et al., “THE DETECTION AND MEASUREMENT OF INFRARED RADIATION,” Oxford at the Clarendon Press, 1968, P195).
The shot noise in2 as vacuum tube noise can be expressed by:in2=Γ2·2e·Ip·Bf where in2=Square Value of Noise Current;
e=Charge of Electron;
Ip=Anode DC Current; and
Bf=Frequency Band of Signal Amplifier;
Γ2 is a decreasing function of a cathode temperature Tk, and is measured as a value in a range of 0.16 to 0.018 (see “Communications Engineering Handbook” edited by Japanese Society of Electronic and Communications Engineers, p471 (1957)).
In regard to detection of signals by an electron beam apparatus, information on the infrared technologies and electron tube technologies as described above are not utilized in an effective way, and the shot noise is treated as Γ=1. In addition, although the shot noise can be reduced by increasing the cathode temperature of the electron gun, the cathode temperature is determined without taking into account the shot noise.